rustbook: Make `main` a public function

This will allow it to be used as a crate in a Cargo-based build

src/rustbook/main.rs

@@ -36,7 +36,7 @@ mod test;
 
 static EXIT_STATUS: AtomicIsize = ATOMIC_ISIZE_INIT;
 
-fn main() {
+pub fn main() {
     let mut term = Term::new();
     let cmd: Vec<_> = env::args().collect();